1. Field of the Invention
The present invention is concerned with assays for screening a sample of body fluid for autoantibodies to various antigens. In particular, the present invention is concerned with screening a sample of body fluid for autoantibodies associated with autoimmune thyroid disease.
2. Description of Related Art
Autoimmune diseases are characterised by the presence of circulating autoantibodies and in autoimmune thyroid disease, for example, the autoantibodies are directed to three different thyroid proteins, namely thyroid peroxidase (TPO), thyroglobulin (Tg) and the receptor for thyroid stimulating hormone (TSHR).
In the absence of thyroid disease, thyroid function is controlled by a feedback system involving the pituitary gland. The pituitary secretes the hormone thyroid stimulating hormone (TSH) into the circulating blood. TSH then acts on TSH receptors (TSHR) on the surfaces of thyroid cells in such a way as to stimulate the synthesis and release of thyroid hormones (which stimulate metabolic processes in almost all cells). As circulating thyroid hormone levels rise these hormones act back on the pituitary to inhibit TSH release. This causes blood TSH levels to fall with the effect of lowering blood thyroid hormone levels. This feedback mechanism allows circulating thyroid hormone levels to be maintained within close limits, thus ensuring good control over metabolic activity.
In thyroid disease, however, the above feedback system is often distorted. For example, when the thyroid is under-active (hypothyroidism), thyroid hormone levels are lower than normal and because these low levels do not suppress TSH release, circulating TSH levels are high. In the case of thyroid over-activity (hyperthyroidism), thyroid levels are higher than normal and these high levels cause TSH release to be suppressed more than normal and circulating TSH levels are low.
Hypothyroidism is often caused by autoimmune attack on the thyroid and this attack is associated with the formation of autoantibodies to two different thyroid proteins (autoantigens), namely TPO and Tg. Screening for autoantibodies to TPO and/or autoantibodies to Tg is important in the diagnosis and management of the various forms of autoimmune hypothyroidism, including post-partum thyroiditis and the like. This screening for TPO autoantibodies and/or Tg autoantibodies (which indicates the likely cause of thyroid underactivity) complements monitoring of circulating TSH levels or thyroid hormone levels which reflect the extent of thyroid under-activity and effectiveness of treatment.
Hyperthyroidism is also often caused by autoimmune attack on the thyroid but in this condition, autoantibodies are formed to the TSHR. These TSHR autoantibodies mimic the effects of TSH and cause circulating thyroid hormone levels to be high. Such high thyroid hormone levels act on the pituitary and suppress circulating TSH levels and consequently TSH levels are lower than normal. Screening for autoantibodies to the TSHR is important in the diagnosis of autoimmune hyperthyroidism. As with autoimmune hypothyroidism, screening for TSHR autoantibodies complements monitoring of circulating TSH levels or thyroid hormone levels which reflect the extent of thyroid over-activity and effectiveness of treatment.
In patients with thyroid cancer, screening for circulating levels of Tg is often used as an indicator of the presence of any residual malignant thyroid tumour cells after treatment. Tg levels are usually measured by assays which depend on monoclonal and/or polyclonal antibodies to Tg but if autoantibodies to Tg are present in patient test samples, these autoantibodies can interfere with the Tg assays, giving erroneous results. Consequently, screening for autoantibodies to Tg is often carried out at the same time as detection and monitoring of circulating Tg levels.
Many examples of other (i.e. non thyroid) autoimmune diseases are known, such as type 1 diabetes (where autoantibodies are formed to insulin, glutamic acid decarboxylase and to the islet cell protein ICA512 or IA2), celiac disease (where autoantibodies are formed to tissue transglutaminase), myasthenia gravis (where autoantibodies are formed to the acetylcholine receptor and to calcium channels), systemic lupus erythematosus (where autoantibodies are formed to DNA and to various nuclear proteins), and the like.
Currently, several types of assay have been used to measure autoantibodies. These include methods using, for example, radioactive labels in which the labelled antigen binds directly to the respective autoantibody, or methods using radioactive labels in competition assays. Several non-radioactive assays have also been used, including those based on agglutination of particles coated with antigen. In addition, sandwich type enzyme linked immunosorbent assays (ELISA) are available. These sandwich type enzyme linked immunosorbent assays have used ELISA plates coated with antigen in combination with an anti-human IgG reagent conjugated with an enzyme such as horseradish peroxidase.
However, there have been some major limitations associated with current assay methods for autoantibodies, for example:—                the current assays can only be carried out away from the patient in specially equipped laboratories; and        the current assays can only be carried out by experienced personnel and take several hours to complete.        
It is therefore the aim of the present invention to provide an improved assay system which alleviates some of the aforementioned problems.
It is a further object of the present invention to provide simple and rapid assay methods for the monitoring of autoantibodies and also to provide diagnostic kits for use in the simple and rapid detection of autoantibodies for the diagnosis of autoimmune diseases. It is a further object of the present invention to provide an assay method that can be carried out near the point of patient care by personnel who do not have experience in laboratory procedures.